THE VOYAGE OF THE BEAGLE 353 



scarcely comprehensible as such changes must ever appear, 

 yet they have all occurred within a period, recent when 

 compared with the history of the Cordillera; and the Cor- 

 dillera itself is absolutely modern as compared with many 

 of the fossiliferous strata of Europe and America. 



April ist. We crossed the Upsallata range, and at night 

 slept at the custom-house the only inhabited spot on the 

 plain. Shortly before leaving the mountains, there was a 

 very extraordinary view; red, purple, green, and quite white 

 sedimentary rocks, alternating with black lavas, were broken 

 up and thrown into all kinds of disorder by masses of por- 

 phyry of every shade of colour, from dark brown to the 

 brightest lilac. It was the first view I ever saw, which 

 really resembled those pretty sections which geologists make 

 of the inside of the earth. 



The next day we crossed the plain, and followed the course 

 of the same great mountain stream which flows by Luxan. 

 Here it was a furious torrent, quite impassable, and appeared 

 larger than in the low country, as was the case with the rivu- 

 let of Villa Vicencio. On the evening of the succeeding day, 

 we reached the Rio de las Vacas, which is considered the 

 worst stream in the Cordillera to cross. As all these rivers 

 have a rapid and short course, and are formed by the melting 

 of the snow, the hour of the day makes a considerable differ- 

 ence in their volume. In the evening the stream is muddy 

 and full, but about daybreak it becomes clearer, and much 

 less impetuous. This we found to be the case with the Rio 

 Vacas, and in the morning we crossed it with little difficulty. 



The scenery thus far was very uninteresting, compared 

 with that of the Portillo pass. Little can be seen beyond the 

 bare walls of the one grand flat-bottomed valley, which the 

 road follows up to the highest crest. The valley and 

 the huge rocky mountains are extremely barren: during the 

 two previous nights the poor mules had absolutely nothing 

 to eat, for excepting a few low resinous bushes, scarcely a 

 plant can be seen. In the course of this day we crossed some 

 of the worst passes in the Cordillera, but their danger has 

 been much exaggerated. I was told that if I attempted to 

 pass on foot, my head would turn giddy, and that there was 

 no room to dismount; but I did not see a place where any 

 VOL. xxix L HC 



